Method and system for measuring and calibrating imaging magnetic field in magnetic resonance apparatus

ABSTRACT

A method and a system for measuring and calibrating an imaging magnetic field in a magnetic resonance apparatus are provided. The method includes: providing the imaging magnetic field, where the imaging magnetic field is adapted for scanning an object; sampling a signal corresponding to the imaging magnetic field; processing the signal to obtain an actual magnetic field intensity; and calibrating based on a difference between the actual magnetic field intensity and a target magnetic field intensity. The system includes: a magnetic component, adapted for scanning an object to be imaged; a sampling unit, adapted for sampling a signal corresponding to the imaging magnetic field; a processing unit, adapted for processing the signal to obtain an actual magnetic field intensity; a calibration unit, adapted for calibrating based on a difference between the actual magnetic field intensity and a target magnetic field intensity; and a control unit, adapted for controlling the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Chinese patent applicationNo. 201310191003.3, filed on May 21, 2013, and entitled “METHOD ANDSYSTEM FOR MEASURING AND CALIBRATING IMAGING MAGNETIC FIELD IN MAGNETICRESONANCE APPARATUS”, the entire disclosure of which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure generally relates to magnetic resonancetechnology, and more particularly, to a method and a system formeasuring and calibrating an imaging magnetic field in a magneticresonance apparatus.

BACKGROUND

A magnetic resonance apparatus generally includes a superconductingcoil, a gradient coil, a radio-frequency (RF) coil, a computer systemand other auxiliary equipments.

The superconducting coil is adapted for generating a main imagingmagnetic field, and the gradient coil is adapted for proving a gradientfield, which can be used to cooperate with the main imaging magneticfield to form an imaging magnetic field (referred to as B₀ field forshort). The RF coil includes a transmitter coil and a receiver coil. Thetransmitter coil is usually a body coil. The transmitter coil transmitsa RF pulse to stimulate protons in human body to resonate. The receivercoil receives a magnetic resonance signal emitted from the human body.The computer system controls pulse excitation, signal sampling, dataoperation, image display and etc.

However, the B₀ filed has disadvantages of non-uniformity and drift.

In magnetic resonance imaging, a high uniform field is beneficial forimproving signal-to-noise ratio of image, ensuring an accurate spatialorientation, reducing artifacts, and improving scan view and etc.

Currently, a plurality of methods for improving uniformity of theimaging magnetic field are provided in practical applications. Forexample, a method for calibrating an imaging magnetic field in amagnetic resonance apparatus is provided in a Chinese patent applicationCN101509964C. The calibration method applies a sequence of navigationecho in data sampling, the navigation echo and an image echo are fromdifferent spatial dimensions, and a phase encoding gradient is appliedonly before sampling the image echo. Therefore, a two dimensional imagedata and a two dimensional navigation data are obtained. Based on thesampled data, an image processing is performed to calibrate thenon-uniformity of the imaging magnetic field.

Besides the non-uniformity, stability is also an important factor toevaluate the imaging magnetic field. The stability of the imagingmagnetic field is classified into thermal stability and time stability.The gradient coil adapted for providing the gradient field releases alot of heat, which would result in temperature rising of the magnet warmbore. Furthermore, the temperature of the magnet warm bore may risebecause of eddy currents, resulting in variation of magneticconductivity, which will further result in drift of the imaging magneticfield and a negative effect on image quality. A method for avoidingmagnetic field drift by controlling temperature variation is provided ina Chinese patent application CN101427919C.

However, the methods mentioned above either require many additionalhardware to correct for anomalies in the imaging magnetic field, orcorrect the image in a post-acquisition process, which may result inpoor quality of images and require further improvement.

SUMMARY

In order to solve the problems mentioned above, a method for measuringand calibrating an imaging magnetic field in a magnetic resonanceapparatus is provided in this disclosure. The method may include:providing the imaging magnetic field, where the imaging magnetic fieldis adapted for scanning an object to be imaged; sampling a signalcorresponding to the imaging magnetic field; processing the signal toobtain an actual magnetic field intensity; and performing calibrationbased on a difference between the actual magnetic field intensity and atarget magnetic field intensity.

In some embodiments, the step of providing the imaging magnetic fieldincludes: providing the imaging magnetic field by a magnetic componentin the magnetic resonance apparatus; the step of sampling a signalcorresponding to the imaging magnetic field includes: providing ameasurement RF signal to stimulate a monitoring sample and generate ameasurement magnetic resonance signal corresponding to the imagingmagnetic field; and sampling the measurement magnetic resonance signal;and the step of processing the signal to obtain an actual magnetic fieldintensity includes: obtaining the actual magnetic field intensityaccording to the measurement magnetic resonance signal based on magneticresonance principle.

In some embodiments, the step of providing a measurement RF signalincludes: providing the measurement RF signal by a probe or a body coilfixed in the magnetic component.

In some embodiments, in the step of providing a measurement RF signal tostimulate a monitoring sample, the monitoring sample generates themeasurement magnetic resonance signal corresponding to the imagingmagnetic field.

In some embodiments, a method for forming the measurement RF signalincludes: adopting protons as the monitoring sample to generate themeasurement RF signal, where the protons are same with or different fromprotons in an imaging process.

In some embodiments, the step of performing calibration includes:calibrating the imaging magnetic field.

In some embodiments, the imaging magnetic field is measured andcalibrated before scanning the object to be imaged.

In some embodiments, the imaging magnetic field is measured andcalibrated in a process of scanning the object to be imaged.

In some embodiments, the step of sampling the measurement magneticresonance signal includes: obtaining the measurement magnetic resonancesignal in a manner of multiple-stimulation and multiple-sampling, or ina manner of single-stimulation and multiple-sampling.

In some embodiments, wherein the step of sampling the measurementmagnetic resonance signal includes: sampling a group of measurementmagnetic resonance signals; and the step of processing the signal toobtain an actual magnetic field intensity includes: establishing aphysical model of the actual magnetic field intensity based on the groupof measurement magnetic resonance signals, so as to predict change ofthe actual magnetic field intensity.

In some embodiments, the step of performing calibration based on adifference between the actual magnetic field intensity and a targetmagnetic field intensity includes: obtaining an imaging magnetic fieldcalibration value based on the difference between the actual magneticfield intensity and the target magnetic field intensity; obtaining acurrent calibration value of the magnetic component according to theimaging magnetic field calibration value; and calibrating the imagingmagnetic field provided by the magnetic component based on the currentcalibration value of the magnetic component, where the differencebetween the actual magnetic field intensity and the target magneticfield intensity is divided into a uniform deviation, a linear deviationand a high-order deviation after calculation.

In some embodiments, the magnetic component includes a gradient coil;the step of obtaining a current calibration value of the magneticcomponent according to the imaging magnetic field calibration valueincludes: obtaining a current calibration value of the gradient coilaccording to the imaging magnetic field calibration value; and the stepof calibrating the imaging magnetic field provided by the magneticcomponent based on the current calibration value of the magneticcomponent includes: calibrating a current of the gradient coil tocalibrate the linear deviation.

In some embodiments, the magnetic component further includes a shimcoil; the step of obtaining a current calibration value of the magneticcomponent according to the imaging magnetic field calibration valueincludes: obtaining a current calibration value of the shim coilaccording to the imaging magnetic field calibration value; and the stepof calibrating the imaging magnetic field provided by the magneticcomponent based on the current calibration value of the magneticcomponent includes: calibrating a current of the shim coil to calibratethe linear deviation and the high-order deviation.

In some embodiments, the magnetic component further includes a driftsupply coil for a main imaging magnetic field; the step of obtaining acurrent calibration value of the magnetic component according to theimaging magnetic field calibration value includes: obtaining a currentcalibration value of the drift supply coil for the main imaging magneticfield according to the imaging magnetic field calibration value; and thestep of calibrating the imaging magnetic field provided by the magneticcomponent based on the current calibration value of the magneticcomponent includes: calibrating a current of the drift supply coil forthe main imaging magnetic field to calibrate the uniform deviation.

In some embodiments, the magnetic component further performs imagereconstruction; and the step of performing calibration based on adifference between the actual magnetic field intensity and a targetmagnetic field intensity includes: performing the image reconstructionaccording to data obtained by the actual magnetic field intensity incombination with the difference between the actual magnetic fieldintensity and the target magnetic field intensity, so as to output acalibrated image.

In some embodiments, the step of sampling a signal corresponding to theimaging magnetic field includes: inducing a change of the imagingmagnetic field to form an alternating electromotive force correspondingto the imaging magnetic field, and sampling the alternatingelectromotive force; and the step of processing the signal to obtain anactual magnetic field intensity includes: obtaining the actual magneticfield intensity according to the alternating electromotive force basedon electromagnetic induction principle.

In one embodiment, a system for measuring and calibrating an imagingmagnetic field in a magnetic resonance apparatus is provided. The systemincludes: a magnetic component, adapted for scanning an object to beimaged; a sampling unit, adapted for sampling a signal corresponding tothe imaging magnetic field; a processing unit, adapted for processingthe signal to obtain an actual magnetic field intensity; a calibrationunit, adapted for performing calibration based on a difference betweenthe actual magnetic field intensity and a target magnetic fieldintensity; and a control unit, connected to the magnetic component, thesampling unit, the processing unit and the calibration unit, and adaptedfor controlling the magnetic component to provide the imaging magneticfield, controlling the sampling unit to sample the signal, controllingthe processing unit to process the signal, and controlling thecalibration unit to calibrate.

In some embodiments, the sampling unit includes a plurality of probesadapted for, after a measurement RF signal stimulates a plurality ofmonitoring samples and generates a measurement magnetic resonance signalcorresponding to the measurement RF signal, sampling the measurementmagnetic resonance signal; a sampling channel adapted for transmittingthe measurement magnetic resonance signal; and the processing unit isadapted for obtaining the actual magnetic field intensity according tothe measurement magnetic resonance signal based on magnetic resonanceprinciple.

In some embodiments, the plurality of monitoring samples is disposed inthe plurality of probes, where the plurality of monitoring samples aresame with or different from a sample which generates an imaging magneticresonance signal.

In some embodiments, the plurality of probes are fixed on a surface ofthe magnetic component.

In some embodiments, the magnetic resonance apparatus further includes alocal coil, and the plurality of probes are embedded in the local coil.

In some embodiments, the plurality of probes are distributed on agradient orthogonal axes.

In some embodiments, the number of the plurality of probes is at leastfour, and the plurality of probes are distributed on three gradientorthogonal axes symmetrically.

In some embodiments, the plurality of probes are transmitting-receivingRF coils, or transmitting RF coils.

In some embodiments, each of the plurality of probes has an adjacentwing at one side.

In some embodiments, the plurality of probes have a solenoid structure.

In some embodiments, a magnetic material is disposed in the plurality ofprobes to generate a local magnetic field to compensate the imagingmagnetic field.

In some embodiments, the local magnetic field changes with a voltageapplied on the magnetic material, so as to calibrate a localdistribution of the imaging magnetic field.

In some embodiments, the magnetic resonance apparatus further includes abody coil adapted for providing the measurement RF signal and an imagingRF signal; the magnetic component includes a gradient coil, and thecontrol unit is connected to the gradient coil, the body coil, theplurality of probes and the sampling channel, where the control unit isadapted for controlling the gradient coil to provide the imagingmagnetic field and controlling the body coil to provide the imaging RFsignal, so as to generate the imaging magnetic resonance signal; and thecontrol unit is further adapted for controlling the body coil to providethe measurement RF signal, controlling the plurality of probes to samplethe measurement magnetic resonance signal, and controlling the samplingchannel to transmit the imaging magnetic resonance signal and themeasurement magnetic resonance signal synchronously, in a process ofmagnetic resonance imaging.

In some embodiments, the sampling channel includes a plurality ofchannels, where a part of the plurality of channels are used to transmitthe imaging magnetic resonance signal, and others are used to transmitthe measurement magnetic resonance signal.

In some embodiments, the magnetic resonance apparatus further includes abody coil adapted for providing the measurement RF signal and an imagingRF signal; the magnetic component includes a gradient coil, and thecontrol unit is connected to the gradient coil, the body coil, theplurality of probes and the sampling channel, where the control unit isadapted for controlling the gradient coil to provide the imagingmagnetic field, controlling the body coil to provide the measurement RFsignal, controlling the plurality of probes to sample the measurementmagnetic resonance signal, and controlling the sampling channel totransmit the measurement magnetic resonance signal, before a process ofmagnetic resonance imaging.

In some embodiments, the control unit controls the body coil tostimulate the monitoring sample once or multiple times, and control theplurality of probes to sample the measurement magnetic resonance signalmultiple times.

In some embodiments, the control unit is adapted for controlling theplurality of probes to sample a group of measurement magnetic resonancesignals, the processing unit is adapted for establishing a physicalmodel of the actual magnetic field intensity based on the group ofmeasurement magnetic resonance signals, so as to predict change of theactual magnetic field intensity.

In some embodiments, the calibration unit includes: a first calculationunit, adapted for obtaining an imaging magnetic field calibration valueaccording to the difference between the actual magnetic field intensityand the target magnetic field intensity; a second calculation unit,adapted for obtaining a current calibration value of the magneticcomponent according to the imaging magnetic field calibration value;where the control unit is connected to the second calculation unit andthe magnetic component, and adapted for changing a current in themagnetic component according to the current calibration value of themagnetic component, so as to calibrate the imaging magnetic field, andthe difference between the actual magnetic field intensity and thetarget magnetic field intensity is divided into a uniform deviation, alinear deviation and a high-order deviation after calculation.

In some embodiments, the magnetic component includes a gradient coil;the first calculation unit is adapted for obtaining an imaging magneticfield calibration value of a linear field according to the differencebetween the actual magnetic field intensity and the target magneticfield intensity; the second calculation unit is adapted for obtaining acurrent calibration value of the gradient coil according to the imagingmagnetic field calibration value of the linear field; and the controlunit is connected to the second calculation unit and the gradient coil,and adapted for calibrating a current of the gradient coil according tothe current calibration value of the gradient coil, so as to calibratethe linear deviation.

In some embodiments, the magnetic component further includes a shim coiladapted for providing a compensation imaging magnetic field; thecalibration unit further includes: a third calculation unit, adapted forobtaining a current calibration value of the shim coil according to theimaging magnetic field calibration value; and the control unit isconnected to the third calculation unit and the shim coil, and adaptedfor changing a current in the shim coil according to the currentcalibration value of the shim coil, so as to calibrate the lineardeviation and the high-order deviation.

In some embodiments, the magnetic component further includes a driftsupply coil for main imaging magnetic field which is adapted forproviding a compensation imaging magnetic field; the calibration unitfurther includes: a fourth calculation unit, adapted for obtaining acurrent calibration value of the drift supply coil for main imagingmagnetic field according to the imaging magnetic field calibrationvalue; and the control unit is connected to the fourth calculation unitand the drift supply coil for main imaging magnetic field, and adaptedfor changing a current in the drift supply coil for main imagingmagnetic field according to the current calibration value of the driftsupply coil for main imaging magnetic field, so as to calibrate theuniform deviation.

In some embodiments, the magnetic component further includes an imagereconstruction unit, where the image reconstruction unit is connected tothe calibration unit, and adapted for reconstructing an image accordingto data obtained by the actual magnetic field intensity in combinationwith the difference between the actual magnetic field intensity and thetarget magnetic field intensity, so as to output a calibrated image.

In some embodiments, the sampling unit includes an induction coiladapted for inducing a change of the imaging magnetic field to form analternating electromotive force corresponding to the imaging magneticfield; and the processing unit is configured to obtain an actualmagnetic field intensity according to the alternating electromotiveforce based on an electromagnetic induction principle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic flow chart of a method for measuring andcalibrating an imaging magnetic field in a magnetic resonance apparatusaccording to one embodiment;

FIG. 2 schematically illustrates a plurality of probes for stimulatingsignals in a step S2 of the method shown in FIG. 1 according to oneembodiment;

FIG. 3 schematically illustrates a measurement RF stimulation signal, ameasurement sampling control signal and a measurement magnetic resonancesignal in the step S2 of the method shown in FIG. 1 according to oneembodiment;

FIG. 4 schematically illustrates a measurement RF stimulation signal, ameasurement sampling control signal and a measurement magnetic resonancesignal in the step S2 of the method shown in FIG. 1 according to oneembodiment;

FIG. 5 schematically illustrates a measurement RF stimulation signal, ameasurement sampling control signal and a measurement magnetic resonancesignal in the step S2 of the method shown in FIG. 1 according to oneembodiment;

FIG. 6 illustrates a schematic waveform diagram of the step S2 and astep S3 of the method shown in FIG. 1 according to one embodiment;

FIG. 7 illustrates a functional block diagram of a system for measuringand calibrating an imaging magnetic field in a magnetic resonanceapparatus according to one embodiment;

FIG. 8 illustrates a schematic structural diagram of a system formeasuring and calibrating an imaging magnetic field in a magneticresonance apparatus according to one embodiment;

FIG. 9 illustrates a schematic structural diagram of a probe shown inFIG. 8 according to one embodiment;

FIG. 10 illustrates a schematic structural diagram of a probe shown inFIG. 8 according to one embodiment;

FIG. 11 illustrates a schematic structural diagram of a probe shown inFIG. 8 according to one embodiment;

FIG. 12 illustrates a schematic structural diagram of a probe shown inFIG. 8 according to one embodiment; and

FIG. 13 illustrates a schematic structural diagram of a probe shown inFIG. 8 according to one embodiment.

DETAILED DESCRIPTION

Magnetic resonance apparatuses require a high uniformity of magneticfield intensity. Besides a certain spatial distribution, the magneticfield intensity may also change with time, which is also known asimaging magnetic field drift. Therefore, the quality of images output byconventional magnetic resonance apparatuses is poor.

In order to clarify the objects, characteristics and advantages of thedisclosure, the embodiments of the present disclosure will be describedin detail in conjunction with the accompanying drawings.

In embodiments of the present disclosure, a method for measuring andcalibrating an imaging magnetic field of a magnetic resonance apparatusis provided. The method includes: providing an imaging magnetic field,where the imaging magnetic field is adapted for scanning an object to beimaged; sampling a signal corresponding to the imaging magnetic field;processing the signal to obtain an actual magnetic field intensity; andperforming calibration based on a difference between the actual magneticfield intensity and a target magnetic field intensity. According toembodiments of the present disclosure, image quality is improved bymeasuring the actual magnetic field intensity and calibrating theimaging magnetic field for magnetic resonance based on the differencebetween the actual magnetic field intensity and the target magneticfield intensity.

Referring to FIG. 1, FIG. 1 illustrates a schematic flow chart of amethod for measuring and calibrating an imaging magnetic field of amagnetic resonance apparatus according to one embodiment of the presentdisclosure. It should be noted that, the imaging magnetic field ismeasured and calibrated based on magnetic resonance principle in oneembodiment, but the present disclosure is not limited thereto. Otherprinciples, such as electromagnetic induction principle, may also beadopted for measuring and calibrating the imaging magnetic field.Specifically, the method may include:

step S1, providing an imaging magnetic field adapted for scanning anobject to be imaged;

step S2, providing a measurement RF signal to stimulate a monitoringsample to generate a measurement magnetic resonance signal correspondingto the imaging magnetic field, and sampling the measurement magneticresonance signal;

step S3, obtaining an actual magnetic field intensity according to themeasurement magnetic resonance signal based on magnetic resonanceprinciple; and

step S4, performing calibration based on a difference between the actualmagnetic field intensity and a target magnetic field intensity.

The steps of the method are described in detail below.

In step S1, an imaging magnetic field is provided, where the imagingmagnetic field is adapted for scanning an object to be imaged.

Generally, a magnetic resonance apparatus includes a cavity having acylindrical shape, the inside of the cavity serves as a sampling area,and the object to be imaged is disposed in the sampling area. It shouldbe noted that, a magnetic component is disposed in the sidewall of thecavity, and the magnetic component is adapted for providing the imagingmagnetic field, so as to obtain a magnetic resonance signal forgenerating a magnetic resonance image.

In one embodiment, intensity of the imaging magnetic field is measuredin real time at a certain position, in order to calibrate the imagingmagnetic field. A high quality magnetic resonance image may be obtainedafter the imaging magnetic field is calibrated. In some embodiments, ahigh quality magnetic resonance image may be obtained by combining imagereconstruction and calibration of magnetic field intensity, which meansperform calibration of magnetic field intensity in the process of imagereconstruction.

In one embodiment, the magnetic component may include a superconductingcoil and a gradient coil, where the superconducting coil is adapted forgenerating a main imaging magnetic field, and the gradient coil isadapted for generating a gradient field. The imaging magnetic field forobtaining a magnetic resonance signal in the cavity of the magneticresonance apparatus is provided by a body coil and the gradient coiltogether. The distribution of the imaging magnetic field can beexpressed as an equation shown below:

B ₀(t,x,y,z)=B ₀₀(t)+Σ_(n=1) ^(N) B _(n)(t,x ^(n) ,y ^(n) ,z ^(n))

where B₀ represents the imaging magnetic field. Taking X-axis as anexample, an actual magnetic field intensity generated by thesuperconducting coil and the gradient coil can be expressed as anequation: B₀=B′₀₀+GX+aX²+cX³.

It should be noted that, the expression of the magnetic field intensityis only expanded to a third-order term. Higher order terms have littleeffect on the image and more complex active shim coils are required toobtain them. Therefore, the higher order terms are compensated bypassive shimming in practical application.

A target imaging magnetic field to the imaging magnetic field may bedetermined based on properties of the superconducting coil and thegradient coil, such as location, material, quantity and etc., inconjunction with design specifications of image quality. A high qualityimage, which can meet design specifications, can be obtained based onthe target imaging magnetic field. In one embodiment, the target imagingmagnetic field may be expressed as B₀₀.

The target imaging magnetic field may be a standard to calibrate theimaging magnetic field. Taking X-axis as an example, a calibration valueof the imaging magnetic field may be expressed as Equation (1):

Δ=(B′ ₀₀ −B ₀₀)+GX+aX ² +cX ³  Equation (1)

where B′₀₀−B₀₀ represents a uniform deviation, GX represents a lineardeviation, and aX²+cX³ represents a high-order deviation of the imagingmagnetic field. In one embodiment, different calibration targets andmethods can be implemented to calibrate the uniform deviation, the linerdeviation and the high-order deviation.

It should be noted that, the method for providing the imaging magneticfield based on the magnetic component is well known to those skilled inthe art, and is not described in detail herein. It should be also notedthat, the composition of the magnetic component is not described hereand the present disclosure should not be limited thereto.

In step S2, a measurement RF signal is provided to stimulate amonitoring sample to generate a measurement magnetic resonance signalcorresponding to the imaging magnetic field.

In one embodiment, the body coil disposed in the cylindrical cavityprovides an imaging RF signal. The body coil also provides themeasurement RF signal to stimulate the monitoring sample, so as togenerate the measurement magnetic resonance signal corresponding to theimaging magnetic field provided in step S1. The measurement magneticresonance signal may be sampled by a plurality of probes disposed in thecavity.

In some embodiments, if the plurality of probes are transceiver coils,the body coil may only provide the imaging RF signal, while theplurality of probes provide the measurement RF signal and sample themeasurement magnetic resonance signal.

Specifically, the monitoring sample for generating the measurementmagnetic resonance signal may be protons which are same with the imagingmagnetic resonance signal, such as H protons, so as to obtain a goodintegration with the magnetic resonance apparatus. It should be notedthat, the monitoring sample should not be limited thereto. In someembodiments, the monitoring sample may be protons which are differentfrom the imaging magnetic resonance signal, so as to reduceinterferences which may be caused in the imaging process by measurementand calibration.

The plurality of probes may be disposed at a fixed position. Positioninformation of the plurality of probes can be obtained in advance, so asto measure the imaging magnetic field at a certain position. But thepresent disclosure is not limited thereto.

Specifically, the spatial relationships between the plurality of probesmay be critical. If the plurality of probes are disposed on a properposition, fewer probes may be needed to obtain a high qualitymeasurement result.

Referring to FIG. 2, a plurality of probes are schematically illustratedaccording to one embodiment. In one embodiment, calibration of a uniformfield and a linear field is taken as an example. The magnetic fieldintensity has a gradient distribution on three gradient orthogonal axes(x, y and z), respectively. By disposing the plurality of probes 100 onthe three gradient orthogonal axes, only four or more probes 100 areneeded to calibrate the uniform field and the linear field.

The plurality of probes may be disposed in the magnetic component. Forexample, the plurality of probes may be disposed in the gradient coil.In some embodiments, the plurality of probes may be disposed on asurface of the magnetic component, in order to measure magnetic fieldintensity in a wide range.

In one embodiment, stimulating the measurement magnetic resonance signaland sampling the measurement magnetic resonance signal are performed inthe scanning process of the apparatus, which means the measurement ofmagnetic field intensity is synchronous with the imaging process.

Specifically, the step of the magnetic resonance apparatus stimulatingmagnetic resonance signals is based on an imaging sequence in theimaging process. Therefore, a measurement RF signal synchronized withthe imaging RF signal can be added in the imaging sequence. The imagingRF signal is used to stimulate the imaging magnetic resonance signal,while the measurement RF signal is used to stimulate the measurementmagnetic resonance signal, so as to measure the imaging magnetic fieldin real time. In other words, the measurement RF signal is stimulatedand sampled with an existed imaging sequence, which may obtain a goodintegration with a conventional magnetic resonance apparatus andsimplify the measurement and calibration process for the imagingmagnetic field.

Also in step S2, the measurement magnetic resonance signal is sampled. Aplurality of channels, adapted for transmitting magnetic resonancesignals, are set in the magnetic resonance apparatus. After themeasurement magnetic resonance signal is sampled by the probes, a partof the plurality of channels is occupied to transmit the measurementmagnetic resonance signal stimulated by the measurement RF signal, whileother channels continue to transmit the imaging magnetic resonancesignal stimulated by the imaging RF signal.

In some embodiments, a measurement sampling signal synchronized with animaging sampling signal is added in the imaging sequence, in order toobtain the measurement magnetic resonance signal based on themeasurement sampling signal in the process of obtaining the imagingmagnetic resonance signal by sampling the imaging sampling signal.

It should be noted that, in step S2, a multiple-stimulation andmultiple-sampling manner may be adopted. As shown in FIG. 3, in ameasurement cycle, four measurement RF stimulation signals 111 are set,and four measurement control sampling signals 112 are setcorrespondingly, so as to obtain four measurement magnetic resonancesignals 114.

Similarly, a single-stimulation and multiple-sampling manner may beadopted. As shown in FIG. 4, in a measurement cycle, three measurementcontrol sampling signals are set corresponding to one measurement RFstimulation signal, so as to obtain three measurement magnetic resonancesignals.

It should be noted that, both FIG. 3 and FIG. 4 schematically illustratea sampling process for frequency information of a measurement magneticresonance signal, but the present disclosure should not be limitedthereto. The measurement magnetic resonance signal also includes phaseinformation. In some embodiments, the phase information of themeasurement magnetic resonance signal may be sampled.

In step S3, an actual magnetic field intensity is obtained according tothe measurement magnetic resonance signal based on magnetic resonanceprinciple.

The measurement magnetic resonance signal includes frequency informationand phase information. The actual magnetic field intensity may beobtained based on the frequency information according to an equationshown below:

${B\left( {r,\theta,\phi,t} \right)} = \frac{f\left( {r,\theta,\phi,t} \right)}{\gamma}$

where (r,θ,φ) represents spatial coordinates of a point in polarcoordinates form, and f represents frequency.

An instantaneous actual magnetic field intensity may also be obtainedbased on the phase information. Phase change accumulated in a timeinterval τ of the measurement sampling signal may be expressed as:dφ=∫_(t0) ^(t0+τ)γB(r,θ,φ,t)dt. If τ is short enough, the actualmagnetic field intensity may be obtained according to an equation shownbelow:

${B\left( {r,\theta,\phi,{t\; 0}} \right)} = {\frac{\int_{t\; 0}^{{t\; 0} + \tau}{\gamma \; {B\left( {r,\theta,\phi,t} \right)}\ {t}}}{\tau}.}$

It should be noted that, imaging magnetic field drift is a gradualchange process with time. In the process of performing step S2 and stepS3, a group of measurement magnetic resonance signals sampled atdifferent time points may be obtained in a step of sampling signalscorresponding to the imaging magnetic field.

Specifically, a group of measurement magnetic resonance signals aresampled densely. Afterward, in a step of processing the measurementmagnetic resonance signals to obtain the actual magnetic fieldintensity, a physical model of the actual magnetic field intensity maybe established based on the group of measurement magnetic resonancesignals densely sampled. The physical model may be used to simulatechanges of the actual magnetic field intensity, so as to estimate orpredict a distribution of magnetic field intensity in a particular timeinterval. After the particular time interval, a new group of measurementmagnetic resonance signals will be sampled to modify the physical model.Signal sampling frequency can be reduced and measurement efficiency canbe improved in this manner.

As shown in FIG. 6, RF_monitor 1 and RF_monitor 2, which represent datacorresponding to the measurement magnetic resonance signals obtained bydense sampling, are used to predict a dynamic distribution of theimaging magnetic field within a particular time interval (dT) and toobtain a predicted result. After a time interval of dT, data RF_monitor3 corresponding to measurement magnetic resonance signals is sampled andis used to modify the predicted result of the dynamic distribution ofthe imaging magnetic field. In this manner, the signal samplingfrequency may be reduced and measurement efficiency may be improved.

In step S4, calibration is performed based on a difference between theactual magnetic field intensity and the target magnetic field intensity.Specifically, after obtaining the actual magnetic field intensity instep S3, an imaging magnetic field calibration value is obtained basedon the difference between the actual magnetic field intensity and thetarget magnetic field intensity, then calibration is performed based onthe imaging magnetic field calibration value.

On one hand, the imaging magnetic field is calibrated based on themagnetic field calibration value, whereby the intensity of the imagingmagnetic field after calibration is close to the target magnetic fieldintensity.

In one embodiment, taking the calibration of a current in the gradientcoil as an example, an encoding gradient is modified according to themagnetic field calibration value, and a K-space trajectory of themagnetic resonance is calibrated. Specifically, a current calibrationvalue is calculated according to G in Equation (1) which is used tocalculate the measurement magnetic resonance signal, and then thecurrent in the gradient coil is calculated to calibrate the lineardeviation.

In some embodiments, a current calibration value of the shim coil may beobtained based on the magnetic field calibration value. A current in theshim coil is changed based on the current calibration value of the shimcoil, so as to calibrate the linear deviation and the high-orderdeviation.

In some embodiments, a current calibration value in a drift supply coilfor main imaging magnetic field may be obtained based on the magneticfield calibration value. A current in the drift supply coil is changedbased on the current calibration value in the drift supply coil tocalibrate the uniform deviation.

On the other hand, the image reconstruction is performed according todata obtained by the actual magnetic field intensity in combination withthe difference between the actual magnetic field intensity and a targetmagnetic field intensity, so as to calibrate the deviations in a finallyobtained image.

Heretofore, calibration of the magnetic field intensity in real time isachieved. Therefore, a high quality image is obtained after calibration,and the measurement accuracy is improved.

It should be noted that, in an actual magnetic resonance imagingprocess, information to control an entire scanning process is configuredin a scanning sequence for magnetic resonance imaging, while informationof the measurement and calibration process is configured in ameasurement calibration sequence. While the scanning sequence ofmagnetic resonance imaging is run, the measurement calibration sequenceis also run. The measurement calibration sequence includes informationof the measurement RF signal for stimulating the monitoring sample, themeasurement control signal for controlling the sampling time andfrequency for the probes and sampling channels to sample the measurementmagnetic resonance signals generated by the monitoring sample.

It should also be noted that, in the above embodiments, the steps ofstimulating and sampling the measurement magnetic resonance signal aresynchronized with the steps of stimulating and sampling the imagingmagnetic resonance signals. In some embodiments, the imaging magneticfield is provided, measured and calibrated before the magnetic resonanceapparatus images. Specifically, after the imaging magnetic field isprovided, the measurement RF signal is provided to stimulate themeasurement magnetic resonance signals, the measurement magneticresonance signal is sampled to obtain the actual magnetic fieldintensity, and calibration is performed based on the difference betweenthe actual magnetic field intensity and the target magnetic fieldintensity. The imaging process is performed based on a calibratedimaging filed obtained after the calibration, so as to obtain a highquality image.

A calibration method on sequence level is mentioned above, which is “acurrent in the gradient coil is calibrated, an encoding gradient ismodified according to the magnetic field calibration value, and aK-space trajectory of the magnetic resonance is calibrated”. In fact,the calibration of the current in the magnetic components may beperformed as the method mentioned above, which means that an informationof current calibration magnitude is fed back to the imaging sequence tocalibrate current magnitude information of the gradient coil, the shimcoil or the drift supply coil for main imaging magnetic field in theimaging sequence, so as to calibrate current magnitudes of these coils.Meanwhile, a computer may be adopted to calibrate current magnitudes incoils according to the current calibration magnitudes. The method forcalibrating current magnetic is not limited herein.

In some embodiments, a method based on the electromagnetic inductionprinciple may be adopted to measure. Specifically, the method mayinclude: inducing a change in an imaging magnetic field to form analternating electromotive force corresponding to the imaging magneticfield, and sampling the alternating electromotive force; and obtainingan actual magnetic field intensity according to the alternatingelectromotive force based on the electromagnetic induction principle.After the measurement based on the electromagnetic induction principle,a magnetic field calibration, which is similar to the calibration methodmentioned in above embodiments, is performed.

Referring to FIG. 7, a functional block diagram of a system formeasuring and calibrating an imaging magnetic field in a magneticresonance apparatus is illustrated according to one embodiment. Thesystem may include:

a magnetic component 201, adapted for scanning an object to be imaged;

a sampling unit 202, adapted for sampling a signal corresponding to animaging magnetic field;

a processing unit 203, adapted for processing the signal to obtain anactual magnetic field intensity;

a calibration unit 204, adapted for performing calibration based on adifference between the actual magnetic field intensity and a targetmagnetic field intensity;

a control unit 205, connected to the magnetic component 201, thesampling unit 202, the processing unit 203 and the calibration unit 204,and adapted for controlling the magnetic component 201 to provide theimaging magnetic field, controlling the sampling unit 202 to sample thesignal, controlling the processing unit 203 to process the signal, andcontrolling the calibration unit 204 to calibrate.

The system for measuring and calibrating an imaging magnetic fieldprovided in embodiments of the present disclosure may measure andcalibrate the imaging field in the magnetic resonance apparatus in realtime, whereby the magnetic resonance apparatus after calibration mayoutput a high quality image.

Referring to FIG. 8, a schematic structural diagram of a system formeasuring and calibrating an imaging magnetic field in a magneticresonance apparatus is illustrated according to one embodiment. Theimaging magnetic field measurement and calibration in this embodiment isbased on the magnetic resonance principle, but it is not limited herein.

Specifically, the magnetic resonance apparatus includes a cavity 210having a cylindrical shape, where an interior region of the cavity 210serves as a sampling area, an object to be magnetic resonance imaged isconfigured to be disposed in the sampling area.

In one embodiment, the system for imaging magnetic field measurement andcalibration in the magnetic resonance apparatus includes a magneticcomponent 212 disposed on an inner sidewall of the cavity 210, where themagnetic component 212 is adapted for providing an imaging magneticfield.

In one embodiment, the magnetic component 212 includes a superconductingcoil (not shown) and a gradient coil 219, where the imaging magneticfield in the magnetic resonance apparatus is provided by thesuperconducting coil and the gradient coil 219.

Specifically, taking an X-axis as an example, an actual magnetic fieldintensity generated by the superconducting coil and the gradient coilcan be expressed as an equation shown below:

B ₀ =B′ ₀₀ +GX+aX ² +cX ³.

It should be noted that, the expression of the magnetic field intensityis only expanded to a third-order term. Higher order terms have littleeffect on the image and more complex active shim coils are required toobtain them. Therefore, the higher order terms are compensated bypassive shimming in practical application.

In one embodiment, the sampling unit 202 may include a plurality ofprobes 211 adapted for, after a measurement RF signal stimulates amonitoring sample and generates a measurement magnetic resonance signalcorresponding to the measurement RF signal, receiving the measurementmagnetic resonance signal.

In one embodiment, the plurality of probes 211 are fixed on a surface ofthe magnetic component 212, so as to measure magnetic field intensity ina wide range. But the locations of the probes 211 are not limitedherein. In some embodiments, the plurality of probes 211 may be fixed onother mechanical structures in the cavity 210. Because the plurality ofprobes 211 are disposed on fixed positions, position information of theplurality of probes 211 can be obtained in advance, and there is no needto detect the position information of the plurality of probes 211 in ameasurement process. But it is not limited herein. In some embodiments,the plurality of probes 211 may be disposed in a non-fixed manner. Insome embodiments, the magnetic resonance apparatus may include a localcoil, and the plurality of probes 211 may be embedded in the local coil.

Referring to FIGS. 9-13, schematic structural diagrams of a probe shownin FIG. 8 are illustrated. As shown in FIG. 9, a probe 2111 may be anannular coil. In some embodiments, as shown in FIG. 10, a probe 2112 maybe a coil having an adjacent wing at one side. In some embodiments, asshown in FIG. 11, a probe 2113 may be a coil having adjacent wings attwo sides. A coil having adjacent wings has a better ability to resistnon-measurement magnetic resonance signals.

In one embodiment, the probe 211 may be a receiving coil. Specifically,as shown in FIG. 12, a probe 2114 may be a solenoid-structure coil. Insome embodiments, as shown in FIG. 13, a probe 2115 may be asolenoid-structure coil capable of resisting outside interferences,which has a similar principle to the above coils having adjacent wings.

It should be noted that, FIGS. 9-13 show a plurality of implementationsof the probe 211, but structure and material of the probe 211 is notlimited in this disclosure.

In some embodiments, a magnetic material is disposed in the probe 211,where the magnetic material is adapted for generating a local magneticfield to compensate the imaging magnetic field. The local magnetic fieldmay change with a voltage applied on the magnetic material, so as tocalibrate a local distribution of the imaging magnetic field. In otherwords, magnetism of the magnetic material is changed by changingdirection of a voltage applied on two ends of the magnetic material, soas to calibrate the local distribution of the imaging magnetic field andto obtain a uniform imaging magnetic field.

In one embodiment, the probe 211 may be a receiving coil, and the systemfurther includes a body coil adapted for providing a measurement RFsignal and an imaging RF signal. But the present disclosure is notlimited hereto. In some embodiments, the probe may have functions ofboth transmitting and receiving RF signals. The probe may provide themeasurement RF signal and receive the measurement magnetic resonancesignal, while the body coil may only provide the imaging RF signal.

It should be also noted that, the magnetic resonance apparatus usuallyadopts H protons to generate magnetic resonance. In one embodiment, theprobe may adopt protons (H protons), which is same as the imagingmagnetic resonance signal, to generate magnetic resonance. But thepresent disclosure is not limited thereto. In some embodiments, theprobe may adopt a kind of protons different from the protons adopted bythe magnetic resonance apparatus in the process of imaging.

By adjusting the spatial relationship between the probes in the magneticresonance apparatus, fewer probes may be needed to obtain a high qualitymeasurement result. Specifically, calibration of a uniform field and alinear field is taken as an example. The magnetic field intensity has agradient distribution on three gradient orthogonal axes (x, y and z),respectively. By disposing the plurality of probes on the three gradientorthogonal axes in a symmetrical distribution manner, only four or moreprobes are needed to calibrate the uniform field and the linear field(as shown in FIG. 2).

In one embodiment, the sampling unit further includes a sampling channel215, adapted for transmitting the measurement magnetic resonancesignals. It should be noted that, a plurality of channels are set in aconventional magnetic resonance apparatus. A part of the plurality ofchannels may be used to transmit imaging magnetic resonance signals,while other channels may be used to transmit measurement magneticresonance signals. Therefore, the sampling channel adopts existingcomponents (channels) in the conventional magnetic resonance apparatusto transmit measurement magnetic resonance signals, thus a goodcompatibility with the conventional magnetic resonance apparatus may beobtained and structure of the system for imaging magnetic fieldmeasurement and calibration may be simplified.

In one embodiment, the system further includes a control unit 214 whichis connected to the body coil, the gradient coil 219, the probe 211, thesampling channel 215 and the processing unit 216. The control unit 214is adapted for controlling the body coil to provide the measurement RFsignal to stimulate the monitoring sample, controlling the probe 211 tosample the measurement magnetic resonance signal, controlling thesampling channel 215 to transmit the measurement magnetic resonancesignal, and controlling the processing unit 216 to process themeasurement magnetic resonance signal, so as to measure the imagingmagnetic field.

It should be noted that, information to control the entire measurementand calibration processes is configured in a measurement and calibrationsequence. The control unit 214 runs a magnetic resonance imagingsequence and the measurement and calibration sequence at the same time,and control a time interval, frequency for stimulating the monitoringsample. In some embodiments, the control unit 214 may control themeasurement RF signal to stimulate the monitoring sample repeatedly, andcontrol the probe 211 to sample the measurement magnetic resonancesignal repeatedly (e.g., adopting signals shown in FIG. 3 to obtainmultiple stimulations and multiple samplings). In some embodiments, thecontrol unit 214 may control the measurement RF signal to stimulate themonitoring sample once, and control the probe 211 to sample themeasurement magnetic resonance signal repeatedly (e.g., adopting signalsshown in FIG. 4 to obtain one stimulation and multiple samplings).

It should be noted that, in some embodiments, before stimulating theimaging magnetic resonance signal, the control unit 214 may control thesuperconducting coil and the gradient coil to provide the imagingmagnetic field, control the body coil to stimulate the monitoring sampleto generate the measurement magnetic resonance signal, control the probe211 to sample the measurement magnetic resonance signal, and control thesampling channel to transmit the measurement magnetic resonance signal,so as to calibrate the imaging magnetic field before the magneticresonance apparatus start to scan.

It should be noted that, the measurement magnetic resonance signalincludes frequency information (as shown in FIG. 3 and FIG. 4) and alsophase information (as shown in FIG. 5).

In one embodiment, the system for imaging magnetic field measurement andcalibration further include a processing unit 216 which is adapted forobtaining actual magnetic field intensity according to the measurementmagnetic resonance signal.

The processing unit 216 can obtain the actual magnetic field intensityaccording to frequency of the measurement magnetic resonance signal,which can be expressed as an equation shown below:

${B\left( {r,\theta,\phi,t} \right)} = \frac{f\left( {r,\theta,\phi,t} \right)}{\gamma}$

where (r,θ,α) represents spatial coordinates of a point in polarcoordinates form, and f represents frequency.

The processing unit 216 also can obtain an instantaneous actual magneticfield intensity according to phase information. Phase change accumulatedin a time interval τ of the measurement sampling signal may be expressedas: dφ=∫_(t0) ^(t0+τ)γB(r,θ,φ,t)dt. If τ is short enough, the actualmagnetic field intensity may be obtained according to an equation shownbelow:

${B\left( {r,\theta,\phi,{t\; 0}} \right)} = \frac{\int_{t\; 0}^{{t\; 0} + \tau}{\gamma \; {B\left( {r,\theta,\phi,t} \right)}\ {t}}}{\tau}$

In some embodiments, the processing unit 216 can estimate the actualmagnetic field intensity by according to other information of themeasurement magnetic resonance signal, which is not limited in thisdisclosure.

Imaging magnetic field drift is a gradual change process with time. Thecontrol unit 214 is adapted for controlling the sampling unit to obtaina group of measurement magnetic resonance signals sampled at differenttime points. The processing unit 216 is adapted to establish a physicalmodel of the actual magnetic field intensity based on the group ofmeasurement magnetic resonance signals, in order to predict a change ofthe actual magnetic field intensity. Therefore, there is no need tosample data within a particular time interval, because the processingunit can obtain the imaging magnetic field within the particular timeinterval base on the physical model. After the particular time interval,a new group of measurement magnetic resonance signals will be sampled tomodify the physical model. Therefore, signal sampling frequency can bereduced and measurement efficiency can be improved in this manner.

The system further includes a calibration unit 217. Under control of thecontrol unit 216, the calibration unit is adapted for calibrating theimaging magnetic field in the magnetic resonance apparatus according tothe actual magnetic field intensity obtained by the processing unit 216.

In one embodiment, the calibration unit 217 includes a first calculationunit (not shown) adapted for obtaining an imaging magnetic fieldcalibration value according to a difference between the actual magneticfield intensity obtained by the processing unit 216 and a targetmagnetic field intensity, where the target magnetic field intensity ispredetermined in the first calculation unit and the first calculationunit is connected to the processing unit 216.

It should be noted that, the target magnetic field intensity set in thefirst calculation unit is determined based on location, material, andquantity of the superconducting coil and the gradient coil and designspecifications of image quality. A high quality image, which can meetdesign specifications, can be obtained based on the target imagingmagnetic field. In one embodiment, the target imaging magnetic field maybe expressed as B₀₀.

The first calculation unit may obtain the imaging magnetic fieldcalibration value Δ according to an equation shown below:

Δ=(B′ ₀₀ −B ₀₀)+GX+aX ² +cX ³

where B′₀₀−B₀₀ represents a uniform deviation, GX represents a lineardeviation, and X²+cX³ represents a high-order deviation of the imagingmagnetic field.

The calibration unit further includes a second calculation unit which isadapted for obtaining a current calibration value of magnetic componentaccording to the imaging magnetic field calibration value.

The control unit 214 is connected to the calibration unit 217 and themagnetic components 214, and is adapted for controlling the current of acoil in the magnetic component 214 to calibrate the imaging magneticfield.

Specifically, different calibration targets and methods can be employedto calibrate the uniform deviation, the liner deviation and thehigh-order deviation. For example, after calculating G, which needs tobe calibrated, in Equation (1) according to the measurement magneticresonance signal, a current calibration value is calculated according toG. Assuming a calibration target is the current in the gradient coil, acurrent calibration value corresponding to the gradient coil needs to becalculated.

The control unit 214 is connected to the gradient coil 219, and isadapted for changing the current in the gradient coil 219 according tothe current calibration value of the gradient coil 219, so as tocalibrate the liner field.

It should be noted that, in some embodiments, a shim coil 213 adaptedfor providing a compensation imaging magnetic field is disposed in themagnetic resonance apparatus. The compensation imaging magnetic fieldcooperates with the imaging magnetic field provided by the magneticcomponent 212 to obtain a more uniform imaging magnetic field.

Correspondingly, besides the first calculation unit adapted forobtaining the imaging magnetic field calibration value according to adifference between the actual magnetic field intensity and a targetmagnetic field intensity, the calibration unit 217 further includes athird calculation unit which is adapted for obtaining a currentcalibration value of the shim coil 213 according to the imaging magneticfield calibration value.

The control unit 214 is also connected to the shim coil 213, and isadapted for calibrating the current in the shim coil 213 according tothe current calibration value of the shim coil 213, so as to calibratethe liner deviation and the high-order deviation.

In some embodiments, the magnetic resonance apparatus further include adrift supply coil for main imaging magnetic field (referred to as B₀coil for short) which is adapted for calibrating a drift of the mainimaging magnetic field.

Besides the first calculation unit adapted for obtaining the imagingmagnetic field calibration value according to the difference between theactual magnetic field intensity and the target magnetic field intensity,the calibration unit 217 further includes a fourth calculation unitwhich is adapted for obtaining a current calibration value of the driftsupply coil for main imaging magnetic field according to the imagingmagnetic field calibration value.

The control unit 214 is also connected to the drift supply coil for mainimaging magnetic field, and is adapted for calibrating the current inthe drift supply coil for main imaging magnetic field according to thecurrent calibration value of the drift supply coil for main imagingmagnetic field, so as to calibrate the uniform deviation.

It should be noted that, the control unit 214, the processing unit 216and the calibration unit 217 may be integrated in a computer inpractical applications.

The embodiments described above may calibrate an imaging magnetic field.The imaging magnetic field after calibration is more uniform and stable,whereby a high quality image may be obtained. But the scope of thepresent disclosure should not be limited thereto. In some embodiments,the system further includes an image reconstruction unit, which isconnected to the calibration unit and is adapted for performing imagereconstruction in combination with the difference between the actualmagnetic field intensity and the target magnetic field intensityprovided by the calibration unit, so as to output a calibrated and highquality image.

It should be noted that, the embodiments described above performmeasurement based on the magnetic resonance principle. But the scope ofthe present disclosure should not be limited thereto. Theelectromagnetic induction principle may be used to measure the imagingmagnetic field.

Specifically, the sampling unit may include an induction coil adaptedfor inducing a change in an imaging magnetic field to form analternating electromotive force corresponding to the imaging magneticfield; and a processing unit adapted for obtaining an actual magneticfield intensity according to the alternating electromotive force basedon the electromagnetic induction principle. After the measurementperformed based on the electromagnetic induction principle, a magneticfield calibration, which is similar to the calibration method mentionedin the above embodiments, is performed. Those skilled in the art canmodify and vary the system according to embodiments mentioned above.

Although the present disclosure has been disclosed above with referenceto preferred embodiments thereof, it should be understood that thedisclosure is presented by way of example only, and not limitation.Those skilled in the art can modify and vary the embodiments withoutdeparting from the spirit and scope of the present disclosure.

What is claimed is:
 1. A method for measuring and calibrating an imagingmagnetic field in a magnetic resonance apparatus, comprising: providingthe imaging magnetic field, where the imaging magnetic field is adaptedfor scanning an object to be imaged; sampling a signal corresponding tothe imaging magnetic field; processing the signal to obtain an actualmagnetic field intensity; and performing calibration based on adifference between the actual magnetic field intensity and a targetmagnetic field intensity.
 2. The method according to claim 1, whereinthe step of providing the imaging magnetic field comprises: providingthe imaging magnetic field by a magnetic component in the magneticresonance apparatus; the step of sampling a signal corresponding to theimaging magnetic field comprises: providing a measurement RF signal tostimulate a monitoring sample and generate a measurement magneticresonance signal corresponding to the imaging magnetic field; andsampling the measurement magnetic resonance signal; and the step ofprocessing the signal to obtain an actual magnetic field intensitycomprises: obtaining the actual magnetic field intensity according tothe measurement magnetic resonance signal based on magnetic resonanceprinciple.
 3. The method according to claim 2, wherein the step ofproviding a measurement RF signal comprises: providing the measurementRF signal by a probe or a body coil fixed in the magnetic component. 4.The method according to claim 3, wherein in the step of providing ameasurement RF signal to stimulate a monitoring sample, the monitoringsample generates the measurement magnetic resonance signal correspondingto the imaging magnetic field.
 5. The method according to claim 2,wherein a method for forming the measurement RF signal comprises:adopting protons as the monitoring sample to generate the measurement RFsignal, where the protons are same with or different from protons in animaging process.
 6. The method according to claim 2, wherein the step ofperforming calibration comprises: calibrating the imaging magneticfield.
 7. The method according to claim 6, wherein the imaging magneticfield is measured and calibrated before scanning the object to beimaged.
 8. The method according to claim 6, wherein the imaging magneticfield is measured and calibrated in a process of scanning the object tobe imaged.
 9. The method according to claim 2, wherein the step ofsampling the measurement magnetic resonance signal comprises: obtainingthe measurement magnetic resonance signal in a manner ofmultiple-stimulation and multiple-sampling, or in a manner ofsingle-stimulation and multiple-sampling.
 10. The method according toclaim 2, wherein the step of sampling the measurement magnetic resonancesignal comprises: sampling a group of measurement magnetic resonancesignals; and the step of processing the signal to obtain an actualmagnetic field intensity comprises: establishing a physical model of theactual magnetic field intensity based on the group of measurementmagnetic resonance signals, so as to predict change of the actualmagnetic field intensity.
 11. The method according to claim 6, whereinthe step of performing calibration based on a difference between theactual magnetic field intensity and a target magnetic field intensitycomprises: obtaining an imaging magnetic field calibration value basedon the difference between the actual magnetic field intensity and thetarget magnetic field intensity; obtaining a current calibration valueof the magnetic component according to the imaging magnetic fieldcalibration value; and calibrating the imaging magnetic field providedby the magnetic component based on the current calibration value of themagnetic component, where the difference between the actual magneticfield intensity and the target magnetic field intensity is divided intoa uniform deviation, a linear deviation and a high-order deviation aftercalculation.
 12. The method according to claim 11, wherein the magneticcomponent comprises a gradient coil; the step of obtaining a currentcalibration value of the magnetic component according to the imagingmagnetic field calibration value comprises: obtaining a currentcalibration value of the gradient coil according to the imaging magneticfield calibration value; and the step of calibrating the imagingmagnetic field provided by the magnetic component based on the currentcalibration value of the magnetic component comprises: calibrating acurrent of the gradient coil to calibrate the linear deviation.
 13. Themethod according to claim 12, wherein the magnetic component furthercomprises a shim coil; the step of obtaining a current calibration valueof the magnetic component according to the imaging magnetic fieldcalibration value comprises: obtaining a current calibration value ofthe shim coil according to the imaging magnetic field calibration value;and the step of calibrating the imaging magnetic field provided by themagnetic component based on the current calibration value of themagnetic component comprises: calibrating a current of the shim coil tocalibrate the linear deviation and the high-order deviation.
 14. Themethod according to claim 12, wherein the magnetic component furthercomprises a drift supply coil for a main imaging magnetic field; thestep of obtaining a current calibration value of the magnetic componentaccording to the imaging magnetic field calibration value comprises:obtaining a current calibration value of the drift supply coil for themain imaging magnetic field according to the imaging magnetic fieldcalibration value; and the step of calibrating the imaging magneticfield provided by the magnetic component based on the currentcalibration value of the magnetic component comprises: calibrating acurrent of the drift supply coil for the main imaging magnetic field tocalibrate the uniform deviation.
 15. The method according to claim 13,wherein the magnetic component further comprises a drift supply coil fora main imaging magnetic field; the step of obtaining a currentcalibration value of the magnetic component according to the imagingmagnetic field calibration value comprises: obtaining a currentcalibration value of the drift supply coil for the main imaging magneticfield according to the imaging magnetic field calibration value; and thestep of calibrating the imaging magnetic field provided by the magneticcomponent based on the current calibration value of the magneticcomponent comprises: calibrating a current of the drift supply coil forthe main imaging magnetic field to calibrate the uniform deviation. 16.The method according to claim 2, wherein the magnetic component furtherperforms image reconstruction; and the step of performing calibrationbased on a difference between the actual magnetic field intensity and atarget magnetic field intensity comprises: performing the imagereconstruction according to data obtained by the actual magnetic fieldintensity in combination with the difference between the actual magneticfield intensity and the target magnetic field intensity, so as to outputa calibrated image.
 17. The method according to claim 1, wherein thestep of sampling a signal corresponding to the imaging magnetic fieldcomprises: inducing a change of the imaging magnetic field to form analternating electromotive force corresponding to the imaging magneticfield, and sampling the alternating electromotive force; and the step ofprocessing the signal to obtain an actual magnetic field intensitycomprises: obtaining the actual magnetic field intensity according tothe alternating electromotive force based on electromagnetic inductionprinciple.
 18. A system for measuring and calibrating an imagingmagnetic field in a magnetic resonance apparatus, comprising: a magneticcomponent, adapted for scanning an object to be imaged; a sampling unit,adapted for sampling a signal corresponding to the imaging magneticfield; a processing unit, adapted for processing the signal to obtain anactual magnetic field intensity; a calibration unit, adapted forperforming calibration based on a difference between the actual magneticfield intensity and a target magnetic field intensity; and a controlunit, connected to the magnetic component, the sampling unit, theprocessing unit and the calibration unit, and adapted for controllingthe magnetic component to provide the imaging magnetic field,controlling the sampling unit to sample the signal, controlling theprocessing unit to process the signal, and controlling the calibrationunit to calibrate.
 19. The system according to claim 18, wherein thesampling unit comprises: a plurality of probes adapted for, after ameasurement RF signal stimulates a plurality of monitoring samples andgenerates a measurement magnetic resonance signal corresponding to themeasurement RF signal, sampling the measurement magnetic resonancesignal; a sampling channel adapted for transmitting the measurementmagnetic resonance signal; and the processing unit is adapted forobtaining the actual magnetic field intensity according to themeasurement magnetic resonance signal based on magnetic resonanceprinciple.
 20. The system according to claim 19, wherein the pluralityof monitoring samples is disposed in the plurality of probes, where theplurality of monitoring samples are same with or different from a samplewhich generates an imaging magnetic resonance signal.
 21. The systemaccording to claim 19, wherein the plurality of probes are fixed on asurface of the magnetic component.
 22. The system according to claim 19,wherein the magnetic resonance apparatus further includes a local coil,and the plurality of probes are embedded in the local coil.
 23. Thesystem according to claim 19, wherein the plurality of probes aredistributed on a gradient orthogonal axes.
 24. The system according toclaim 23, wherein the number of the plurality of probes is at leastfour, and the plurality of probes are distributed on three gradientorthogonal axes symmetrically.
 25. The system according to claim 19,wherein the plurality of probes are transmitting-receiving RF coils, ortransmitting RF coils.
 26. The system according to claim 19, whereineach of the plurality of probes has an adjacent wing at one side. 27.The system according to claim 19, wherein the plurality of probes have asolenoid structure.
 28. The system according to claim 19, wherein amagnetic material is disposed in the plurality of probes to generate alocal magnetic field to compensate the imaging magnetic field.
 29. Thesystem according to claim 28, wherein the local magnetic field changeswith a voltage applied on the magnetic material, so as to calibrate alocal distribution of the imaging magnetic field.
 30. The systemaccording to claim 19, wherein the magnetic resonance apparatus furthercomprises a body coil adapted for providing the measurement RF signaland an imaging RF signal; the magnetic component comprises a gradientcoil, and the control unit is connected to the gradient coil, the bodycoil, the plurality of probes and the sampling channel, where thecontrol unit is adapted for controlling the gradient coil to provide theimaging magnetic field and controlling the body coil to provide theimaging RF signal, so as to generate the imaging magnetic resonancesignal; and the control unit is further adapted for controlling the bodycoil to provide the measurement RF signal, controlling the plurality ofprobes to sample the measurement magnetic resonance signal, andcontrolling the sampling channel to transmit the imaging magneticresonance signal and the measurement magnetic resonance signalsynchronously, in a process of magnetic resonance imaging.
 31. Thesystem according to claim 30, wherein the sampling channel comprises aplurality of channels, where a part of the plurality of channels areused to transmit the imaging magnetic resonance signal, and others areused to transmit the measurement magnetic resonance signal.
 32. Thesystem according to claim 19, wherein the magnetic resonance apparatusfurther comprises a body coil adapted for providing the measurement RFsignal and an imaging RF signal; the magnetic component comprises agradient coil, and the control unit is connected to the gradient coil,the body coil, the plurality of probes and the sampling channel, wherethe control unit is adapted for controlling the gradient coil to providethe imaging magnetic field, controlling the body coil to provide themeasurement RF signal, controlling the plurality of probes to sample themeasurement magnetic resonance signal, and controlling the samplingchannel to transmit the measurement magnetic resonance signal, before aprocess of magnetic resonance imaging.
 33. The system according to claim30, wherein the control unit controls the body coil to stimulate themonitoring sample once or multiple times, and control the plurality ofprobes to sample the measurement magnetic resonance signal multipletimes.
 34. The system according to claim 32, wherein the control unitcontrols the body coil to stimulate the monitoring sample once ormultiple times, and control the plurality of probes to sample themeasurement magnetic resonance signal multiple times.
 35. The systemaccording to claim 19, wherein the control unit is adapted forcontrolling the plurality of probes to sample a group of measurementmagnetic resonance signals, the processing unit is adapted forestablishing a physical model of the actual magnetic field intensitybased on the group of measurement magnetic resonance signals, so as topredict change of the actual magnetic field intensity.
 36. The systemaccording to claim 19, wherein the calibration unit comprises: a firstcalculation unit, adapted for obtaining an imaging magnetic fieldcalibration value according to the difference between the actualmagnetic field intensity and the target magnetic field intensity; asecond calculation unit, adapted for obtaining a current calibrationvalue of the magnetic component according to the imaging magnetic fieldcalibration value; where the control unit is connected to the secondcalculation unit and the magnetic component, and adapted for changing acurrent in the magnetic component according to the current calibrationvalue of the magnetic component, so as to calibrate the imaging magneticfield, and the difference between the actual magnetic field intensityand the target magnetic field intensity is divided into a uniformdeviation, a linear deviation and a high-order deviation aftercalculation.
 37. The system according to claim 36, wherein the magneticcomponent comprises a gradient coil; the first calculation unit isadapted for obtaining an imaging magnetic field calibration value of alinear field according to the difference between the actual magneticfield intensity and the target magnetic field intensity; the secondcalculation unit is adapted for obtaining a current calibration value ofthe gradient coil according to the imaging magnetic field calibrationvalue of the linear field; and the control unit is connected to thesecond calculation unit and the gradient coil, and adapted forcalibrating a current of the gradient coil according to the currentcalibration value of the gradient coil, so as to calibrate the lineardeviation.
 38. The system according to claim 37, wherein the magneticcomponent further comprises a shim coil adapted for providing acompensation imaging magnetic field; the calibration unit furthercomprises: a third calculation unit, adapted for obtaining a currentcalibration value of the shim coil according to the imaging magneticfield calibration value; and the control unit is connected to the thirdcalculation unit and the shim coil, and adapted for changing a currentin the shim coil according to the current calibration value of the shimcoil, so as to calibrate the linear deviation and the high-orderdeviation.
 39. The system according to claim 37, wherein the magneticcomponent further comprises a drift supply coil for main imagingmagnetic field which is adapted for providing a compensation imagingmagnetic field; the calibration unit further comprises: a fourthcalculation unit, adapted for obtaining a current calibration value ofthe drift supply coil for main imaging magnetic field according to theimaging magnetic field calibration value; and the control unit isconnected to the fourth calculation unit and the drift supply coil formain imaging magnetic field, and adapted for changing a current in thedrift supply coil for main imaging magnetic field according to thecurrent calibration value of the drift supply coil for main imagingmagnetic field, so as to calibrate the uniform deviation.
 40. The systemaccording to claim 38, wherein the magnetic component further comprisesa drift supply coil for main imaging magnetic field which is adapted forproviding a compensation imaging magnetic field; the calibration unitfurther comprises: a fourth calculation unit, adapted for obtaining acurrent calibration value of the drift supply coil for main imagingmagnetic field according to the imaging magnetic field calibrationvalue; and the control unit is connected to the fourth calculation unitand the drift supply coil for main imaging magnetic field, and adaptedfor changing a current in the drift supply coil for main imagingmagnetic field according to the current calibration value of the driftsupply coil for main imaging magnetic field, so as to calibrate theuniform deviation.
 41. The system according to claim 19, wherein themagnetic component further comprises an image reconstruction unit, wherethe image reconstruction unit is connected to the calibration unit, andadapted for reconstructing an image according to data obtained by theactual magnetic field intensity in combination with the differencebetween the actual magnetic field intensity and the target magneticfield intensity, so as to output a calibrated image.
 42. The systemaccording to claim 18, wherein the sampling unit comprises an inductioncoil adapted for inducing a change of the imaging magnetic field to forman alternating electromotive force corresponding to the imaging magneticfield; and the processing unit is configured to obtain an actualmagnetic field intensity according to the alternating electromotiveforce based on an electromagnetic induction principle.